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and sponge denizens suggests that sponges may influence the prokaryote composition of organisms that live on or within them or that feed on them. The sponge denizen biotope included sea cucumbers and barnacles that lived within or on the sponge, presumably for much of their life (Hammond and Wilkinson 1985; van Syoc and Newman 2010). Nudibranchs, however, are more mobile and may represent vectors carrying microorganisms from one sponge to the other. Nudibranchs also come into intimate contact with their sponge prey during feeding whereby certain species evert and extend their pharyngeal bulb deep into the sponge (van Alphen et al. 2011). During this process, they are also able to sequester toxins from the sponge for their own defence (Proksch 1994). Our results indicated that the gut and/or mantle prokaryote communities of specimens from certain nudibranch species (Doriprismatica atromarginata, Phyllidiella pustulosa, Phyllidiella nigra, Phyllidia ocellata and Phyllidia elegans) closely resembled that of sponge prokaryote communities. All of these nudibranch species have been recorded feeding on sponges (Fusetani et al. 1992; Fontana et al. 1999; van Alphen et al. 2011; Wright 2003). A number of these specimens were also collected from sponges while diving. Specimens of the sea cucumber Synaptula sp., a sponge denizen sampled from X. testudinaria, housed a prokaryote community similar to that of the ‘HMA’ sponge cluster, which included X. testudinaria. Members of the genus Synaptula are often common in coral reef habitat, particularly in association with sponges and can sometimes be so abundant that they cover the sponge’s surface. They have also been shown to be able to exploit sponge exudates (Hammond and Wilkinson 1985). Interestingly, the barnacle Acasta sp., which was collected within X. testudinaria, was the only sponge denizen barnacle that also housed a prokaryote community similar to that of members of the ‘HMA’ sponge cluster. The other sponge barnacles were collected within samples of the sponge species Dasychalina fragilis, Agelas cavernosa and Cinachyrella sp.

The similarity between the prokaryote communities of sponges and the guts of certain nudibranch samples may be an indication that the nudibranch gut communities are dominated by transient microorganisms derived from their preferred food source, namely sponges (Proksch 1994; van Alphen et al. 2011). An individual’s diet can have a profound effect on gut prokaryote composition (David et al. 2014; Carmody et al. 2015). This difference can extend to species, whereby there are marked differences in gut microbiome composition among mammal species with different diets (Groussin et al. 2017; Nishida and Ochman 2018). This distinction appears to apply to nudibranchs, whereby the gut and mantle prokaryote communities of species known to feed on sponges closely resembled that of certain sponge species (Fig. 6.3). It would be interesting to test how different diets (e.g. different sponge species) affect the nudibranch prokaryote community.

The very high number of OTUs shared among different biotopes would appear to lend support to the ‘everything is everywhere but the environment selects’ hypothesis of Baas

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Becking (1934). In line with this, the very high richness and evenness of sediment suggest that it may function as a microbial seed bank. There was also considerable compositional similarity between sediment samples and high diversity host samples of algae, stony corals and sea cucumbers among others. In contrast, seawater samples were only compositionally similar to samples of certain sponge species. Previously, Cleary and Polónia (2018) also showed that populations of mussels inhabiting Indonesian marine lakes and mangroves shared much more OTUs with sediment than with seawater and were compositionally more similar to sediment than to seawater. Gibbons et al. (2013) previously suggested that the marine biosphere maintains a persistent microbial seed bank. In their scenario, all microbes are found everywhere due to the immensity and persistence of this seed bank, and apparent local or host-associated endemism is merely a result of insufficient sequencing. Community structure is, thus, a function of relative abundance rather than the presence or absence of certain microbial taxa. The presence of such a seed bank has repercussions for ecological theory, given the limited importance of long-distance dispersal and the ability of low abundance populations to rapidly expand when the appropriate environment is encountered (Gibbons et al. 2013). In the global marine environment, hydrographic parameters of seawater masses, furthermore, greatly contribute to the dispersion of sediment microbial communities at regional and global scales, although microbial cell dispersion is highly dependent on the ability to tolerate stress (Galand et al. 2009; De Rezende et al. 2013). Although wide in scope, the present study only represents a small fraction of marine species in the coral reef environment and even in this dataset, there was considerable variation among species within biotopes. Much more research is needed to understand the variation in microbial composition of taxa such as sea cucumbers, flatworms, algae, and nudibranchs. A large amount of time and resources have been spent studying the prokaryote communities of a limited number of taxa leaving large gaps in our knowledge of the coral reef metacommunity. Sponges have been deemed major contributors to total microbial diversity in the world’s oceans, and are considered to be reservoirs of exceptional microbial diversity2 without, however, having actually studied other host taxa in detail. In coral reefs, sponges do not appear to stand alone as the main contributors to total prokaryote diversity as this study highlights; other biotopes host more diverse prokaryote communities, e.g. sea cucumbers. The present study shows that sponges are only one, albeit an interesting, component of a much larger coral reef metacommunity.

ACKNOWLEDGEMENTS

This work was supported by European Funds through COMPETE [FCOMP-01-0124- FEDER-008657] and by National Funds through the Portuguese Foundation for Science and Technology (FCT) within the LESS CORAL [PTDC/AAC-AMB/115304/2009] and Ecotech- Sponge (PTDC/BIAMIC/6473/2014 – POCI-01-0145-FEDER-016531) projects. This work is also part of the research programmes NWO-VIDI with project number 16.161.301 and ASPASIA

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